# std::ranges::cbegin

From cppreference.com
< cpp‎ | ranges

Defined in header `<ranges>` |  |
Defined in header `<iterator>` |  |
inline namespace /* unspecified */ { inline constexpr /* unspecified */ cbegin = /* unspecified */; } |  | (since C++20) (customization point object)
Call signature |  |
template< class T > requires /* see below */ constexpr /* see below */ auto cbegin( T&& t ); |  | (since C++20)

Returns an iterator to the first element of the const-qualified argument. | (until C++23)
Returns a constant iterator to the first element of the argument. | (since C++23)

Let `CT` be const std::remove_reference_t<T>& if the argument is an lvalue (i.e. `T` is an lvalue reference type), const T otherwise. A call to `ranges::cbegin` is expression-equivalent to ranges::begin(static_cast<CT&&>(t)). | (until C++23)
If the argument is an lvalue or ranges::enable_borrowed_range<std::remove_cv_t<T>> is true, then a call to `ranges::cbegin` is expression-equivalent to: std::const_iterator<decltype(U)>(U) for some expression U equivalent to ranges::begin(`possibly-const-range`(t). In all other cases, a call to `ranges::cbegin` is ill-formed, which can result in substitution failure when the call appears in the immediate context of a template instantiation. | (since C++23)

The return type models std::input_or_output_iterator and `constant-iterator`(since C++23) in all cases.

### Customization point objects

The name `ranges::cbegin` denotes a customization point object, which is a const function object of a literal `semiregular` class type. For exposition purposes, the cv-unqualified version of its type is denoted as `__cbegin_fn`.

All instances of `__cbegin_fn` are equal. The effects of invoking different instances of type `__cbegin_fn` on the same arguments are equivalent, regardless of whether the expression denoting the instance is an lvalue or rvalue, and is const-qualified or not (however, a volatile-qualified instance is not required to be invocable). Thus, `ranges::cbegin` can be copied freely and its copies can be used interchangeably.

Given a set of types `Args...`, if std::declval<Args>()... meet the requirements for arguments to `ranges::cbegin` above, `__cbegin_fn` models

- std::invocable<__cbegin_fn, Args...>,
- std::invocable<const __cbegin_fn, Args...>,
- std::invocable<__cbegin_fn&, Args...>, and
- std::invocable<const __cbegin_fn&, Args...>.

Otherwise, no function call operator of `__cbegin_fn` participates in overload resolution.

### Notes

For an lvalue range e of type T, ranges::cbegin(e) is equivalent to

ranges::begin(std::as_const(e)). | (until C++23)
ranges::begin(e) if T models `constant_range`. Otherwise, ranges::begin(std::as_const(e)) if const T models `constant_range`. Otherwise, std::basic_const_iterator(ranges::begin(e)). | (since C++23)

### Example

Run this code

    #include <cassert>
    #include <ranges>
    #include <vector>

    int main()
    {
        std::vector v{3, 1, 4};
        auto vi = std::ranges::cbegin(v);
        assert(3 == *vi);
        ++vi; // OK, constant-iterator object is mutable
        assert(1 == *vi);
        // *vi = 13; // Error: constant-iterator points to an immutable element

        int a[]{3, 1, 4};
        auto ai = std::ranges::cbegin(a); // cbegin works with C-arrays as well
        assert(3 == *ai and *(ai + 1) == 1);
        // *ai = 13; // Error: read-only variable is not assignable
    }

### See also

ranges::begin (C++20) | returns an iterator to the beginning of a range (customization point object)
begincbegin (C++11)(C++14) | returns an iterator to the beginning of a container or array (function template)

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